1. Field of the Invention
The present invention relates to electromagnetic shielding and heat dissipation for a computer system and more specifically to an apparatus for providing both electromagnetic shielding and heat dissipation for integrated circuits in a computer system.
2. Description of the Related Art
Personal computer systems have attained widespread use. These personal computer systems now provide computing power to many segments of today's modern society. A personal computer system can usually be defined as a desktop, floor-standing, or portable microcomputer that includes a system unit having a central processing unit (CPU) with associated memory, a display panel, a keyboard, a hard disk storage device or other type of storage media such as a floppy disk drive or a compact disk read only memory (CD ROM) drive. These personal computer systems are information handling systems which are designed primarily to give independent computing power to a single user or group of users and are inexpensively priced for purchase by individuals or small businesses.
A portable computer is a self-contained personal computer designed to be easily moved to and operated in various locations. Portable computers are often referred to as laptop, notebook, or subnotebook computers. To be portable, these computers are small, compact, and lightweight. The conventional portable computer includes a base portion and a lid portion that pivotally opens from the base portion when the portable computer is in use. The lid portion contains a flat panel display such as a liquid crystal display (LCD) or other relatively small display.
Notebook computers incorporate electromagnetic shielding to reduce electromagnetic emissions in order to meet regulatory standards on such emissions. Portable computers also use thermal transfer mechanisms (heat-sinks or heat spreaders) to cool the CPU which accounts for almost half the power dissipated within the computer system.
An electromagnetic shield is typically a metallic partition placed between two regions of space. The electromagnetic shield controls the propagation of electric and magnetic fields from one of the regions to the other. An electromagnetic shield may be used to contain electromagnetic fields if the shield surrounds the source of the electromagnetic fields.
A solid electromagnetic shield that completely surrounds a product can be at any potential and still provide effective electromagnetic shielding. That is, the shield prevents outside influences from affecting circuits inside the electromagnetic shield and vice versa. Thus, the electromagnetic shield need not be grounded or have its voltage potential defined in any way. However, an ungrounded or undefined electromagnetic shield should completely enclose the object being protected and that object being protected should have no connection to the outside world.
In practice, however, the electromagnetic shield is not a complete enclosure, and the object inside does have connections to the outside world, either directly, through signal and/or power leads, or indirectly, through stray capacitance due to holes in the electromagnetic shield. In such cases, the electromagnetic shield should be grounded or have its voltage potential defined with respect to the noise source to prevent the noise source's potential from coupling to the enclosed object. An ungrounded or undefined electromagnetic shield's potential varies with conditions and location, and therefore the noise coupled to the object inside also varies.
Grounding also has other benefits. Grounding provides a path for radio frequency (RF) currents to flow on the structure. Grounding also prevents the buildup of AC potentials on the equipment enclosure. Grounding provides a fault-current return path to protect personnel from shock hazards. Grounding also prevents the buildup of static charge.
The electromagnetic shield should have a low-impedance coupling with a voltage reference such as a ground plane of a printed circuit board in at least two places in order to properly define the voltage potential or ground the electromagnetic shield in a computer system. However, today's computer systems include high frequency electromagnetic sources such as processors which may require the electromagnetic shield to be electrically coupled to a voltage reference such as a ground plane at several locations. The higher frequencies of these electromagnetic sources require closer spacings between the grounding connections of the electromagnetic shield to the voltage reference in order to provide effective electromagnetic shielding. Coupling a generally planar electromagnetic shield at several closely spaced locations around its perimeter allows an electromagnetic shield to form the top portion of an effective electromagnetic shield enclosure with a ground plane forming the bottom portion.
Electromagnetic shields constructed from an electrically conductive metal with a high yield strength give the shield resiliency in making electrical contact with an electrically conductive surface electrically coupled to a voltage reference such as a ground plane of a printed circuit board.
Heat distribution has also been a problem with some computers, especially with portable computers. In the past, "heat spreaders" have been used to address this problem. One type of heat spreader is a metal piece that is thermally coupled to a processor and distributes heat away from the processor. Typically, a heat sink is made of relatively pure aluminum for good thermal conductivity and for reduced weight. However, because aluminum is a relatively soft metal with a low yield strength, it makes for a poor electromagnetic shield in that extensions, such as spring fingers, made from aluminum are not resilient enough to continuously provide good electrical contact with another surface. Furthermore, oxide coatings typically form on the outside of items made with aluminum which reduces the ability of the object to provide low impedance electrical connections with other items in contact with the aluminum material. Because heat spreaders typically serve only one function, heat spreaders add extra pieces to the computer system assembly as well as increasing the complexity of the build and repair operations.
Typically, the electromagnetic shielding and the heat dissipation are performed by separate apparatuses. What is need is an apparatus that has the characteristics to be both a good electromagnetic shield and a good heat spreader.